The purpose of the present study is to examine the role of a monoclonal antibody in directing complement-mediated neutralization of parainfluenza virus type 3. This model may provide information about antibody interaction which will be applicable to vaccine development and other areas of research. Even though neither complement nor the monoclonal antibody alone produce significant viral neutralization, together they neutralize the virus. This effect does not appear to be due simply to an increase in the amount of complement bound. Likewise, there is no significant increase in the percentage of C4 that is complexed to C3. Preliminary results suggest that this antibody may direct deposition of C4 to a region of the HN protein which is crucial for lysis of the virus.